Guitar Pro formats lacks backward compatibility so it is not possible to produce .gp4 file using the latest version of the program. Your only option would be to find version 4 of the program and import .gp3 file to it so you can save it as .gp4 file. Without the required version of the program, this conversion is most likely impossible.
Convert Guitar Pro 3 project to Guitar Pro 4 project.
We found 2 software records in the database related to .gp3 to .gp4 file format conversion.
Microsoft Windows software - convert gp3 to gp4 on Windows
Multi-track editor and music composer for guitars
Convert from gp3 file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned gp3 to gp4. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Linux/Unix software - convert gp3 to gp4 on Linux
Open source tablature editor and player